Modeling exercise: currency conversion

convert.php

<?php
declare(strict_types=1);

function convert(string $from, string $to, float $amount)
{
    $exchangeRates = [
        'USD,EUR' => 1.240055,
        // ...
    ];

    $key = $from . ',' . $to;
    if (isset($exchangeRates[$key])) {
        $exchangeRate = $exchangeRates[$key];
    }
    else {
        $alternativeKey = $to . ',' . $from;
        if (isset($exchangeRates[$alternativeKey])) {
            $exchangeRate = 1/$exchangeRates[$alternativeKey];
        } else {
            throw new \RuntimeException('Cannot determine an exchange rate.');
        }
    }

    return $amount * 1/$exchangeRate;
}

// Convert to EUR
var_dump(convert('USD', 'EUR', 10.0));

// Convert back to USD
var_dump(convert('EUR', 'USD', 8.0641584445851));

/*
 * Design issues:
 *
 * - What's the currency of the $amount parameter? What should it be?
 * - What's the currency of the return value? What should it be?
 * - What's the precision of the returned amount? What should it be?
 * - There's way too many levels of indentation. How to fix this?
 * - `1/$exchangeRate` is repeated knowledge about "inverting" an exchange rate. How to solve it?
 * 
 * By means of introducing proper (value) objects, all of these issues will disappear.
 */

See also: https://github.com/matthiasnoback/modeling-exercises